/Engine/Plugins/Editor/FacialAnimation/Source/FacialAnimation/Public/AudioCurveSourceComponent.h
This function allows designers to call a delayed Stop on an Audio Component instance while applying a volume curve over time. Parameters allow designers to indicate the duration of the fade and the curve shape.
virtual void FadeOut ( float FadeOutDuration, float FadeVolumeLevel, const EAudioFaderCurve FadeCurve )